1. FIELD OF THE INVENTION PA1 2. DESCRIPTION OF THE PRIOR ART
The present invention relates to an electromagnetic contactor. More particularly, it relates to an electromagnetic contactor for turning-on and off an electric device such as a motor.
The structure of the typical electromagnetic contactor will be described with reference to FIGS. 1 and 2. The electromagnetic contactor generally comprises a base (10), an arc box (12) and a fitting plate (14) for fitting the contactor to the control panel of an electric apparatus.
The base (10) is made of an insulating material in shape of a box and holds a cross bar (16) and a drive-controller (18) therein. The cross bar (16)includes a vertical wall (16a) extending upwards in the upper central portion and a recess (16b) for holding a movable iron core (described below) at the bottom surface. The cross bar is held and guided by the inner wall of the base so as to be vertically movable. The vertical wall (16a) is provided with an opening (16c) in which a movable contact (20) is placed to be vertically movable but incapable of horizontal movement. A movable contact supporter (22) is placed on the upper surface of the movable contact (20) and a movable contact point (24) is attached at the lower surface of both ends of the movable contact (20) extending from the opening (16c). A spring supporter (25) having a channel shape is provided such a manner that the upper end is engaged with the movable contact supporter and the lower end supports a contactor spring (28). The contact spring (28) is compressed between the stopper (26) projecting from the cross bar (16) and the spring supporter (25) whereby the upper surface of the movable contact supporter (22) and the upper side (25a) of the spring supporter (25) as well as the upper surface of the movable contact (20) and the lower surface of the contact supporter (22) are assembled in a close contact state by the compressive force of the contact spring (28). The movable iron core (30) formed, by example, by piling silicon steel plate is fixed through an elastic plate (32) on the recess (16b) formed in the bottom surface of the cross bar (16) by pins (34).
On the other hand, the drive-controller (18) includes a stationary iron core (36) and a coil holder (38). The stationary iron core is formed, for example, by piling silicon steel plates and pins (40) are horizontally passed through the stationary iron core. A fixed shock-absorbing part (42) is fitted to each projection (40a) of the pins and a guide member (44) is fitted to the fixed shock-absorbing part (42). A fixed shock absorbing spring plate (45) is attached to the bottom surface of the stationary iron core (36). The stationary iron core (36) is put in the through-holes (38a) of the coil holder (38) and an engaging piece (not shown) formed on the coil holder (38) is engaged with a hole (not shown) formed in the guide member (44) to connect the stationary iron core (36) and the coil holder (38) in one piece.
A terminal plate (46) is attached on the upper surface of the base (10) with a screw (48) and a channel-like stationary contact (50) is attached on the upper surface of the terminal plate (46) with a screw (52). A stationary contact point (54) is fixed to the upper surface of the stationary contact (50) so as to face the movable contact point (24). An arc runner (56) is also attached to the terminal plate (46) with a screw (58). A rail plate (60) is attached on the lower surface of the base (10) with a screw (not shown). The rail plate (60) is provided with rail portion (60a) of a channel shape in cross section at both sides of the plate and the guide member (44) is inserted into the rail portion (60a) from the arrow mark direction A in FIG. 1 to prevent the movement of the drive-controller (18) in the vertical direction. After the drive-controller (18) is assembled to the rail plate (60), the rail plate (60) is secured to the bottom surface of the base, thus the stationary iron core (36) is disposed facing the movable iron core (30).
On the outer side surface of the coil holder (38), a ratchet (66) is placed and a compression spring (64) normally urges the ratchet downwards to allow it to move vertically. A pawl (66a) is formed on the ratchet (66) to engage with the engaging hole (60b) of the rail plate (60) so as to restrict the relative movement of the stationary iron core (36) and the coil holder (38) in the arrow mark direction A and vice versa in FIG. 1.
The arc box (12) made of a heat resistant material is fixed to the upper surface of the base (10) by a fastening bolt (68) and a grid (70) and a commutation plate (72) for arc extinction made of a magnetic metal are provided for each pole of R, S and T. Insulation barriers (74) are fixed to the outer side surface of the base (10) to separate the terminal plate (46) for each pole of R, S and T.
A plurality of fitting holes (14a) are formed in the outer periphery of the fitting plate (14) to fix it on a control panel, the fitting plate being attached to the bottom of the rail plate (60) with screws.
A trip spring (76)is placed between the rail plate (60) and the cross bar (16) in a compressed state to normally urge the cross bar (16) in the upward direction whereby the movable contact point (24) and the stationary contact point (54) are in an opening state.
The operation of the electromagnetic contactor having the structure will be described.
When a driving voltage is applied to the coil (78) as a magnetic flux producer which is held by the coil holder (38), an electromagnetic attractive force is produced by magnetic flux between the movable iron core (30) and the stationary iron core (36) to move the cross bar (16) connected to the movable iron core (30) downwards against the trip spring (76) whereby the movable contact (20) is brought into contact with the stationary contact (50) to form an electric circuit. Since the iron core gap G.sub.2 formed between the movable iron core (30) and the stationary iron core (36) is made greater than the contact gap G.sub.1 formed between the movable contact point (24) and the stationary contact point (54), the cross bar (16) is further depressed beyond the contacting position of the contacts until both the iron cores come into contact each other. The contact spring (28) is, therefore, deformed by compression force and the pressure of the spring is transmitted through the spring supporter (25) and the movable contact supporter (22) to the movable contact (20), thus the terminal plates (46), (46) are electrically connected under predetermined contacting pressure.
When the driving voltage applied to the coil (78) disappears, the electromagnetic attracting force acting between the movable iron core (30) and the stationary iron core (36) also disappears and the cross bar (16) is moved upwards by the urging force of the compressed trip spring (76) whereby the movable contact point (24) and the stationary contact point (54) are opened to open the electric circuit. At that moment, arc is produced between the movable contact point (24) and the stationary contact point (54). The arc is transferred from the movable contact point (24) to the commutation plate (72) and also is transferred from the stationary contact point (54) to the arc runner (56) respectively and thus separated arcs enter into gaps between the grids (70) by electromagnetic repulsive force caused by the inter action of arc current and contact current. Thus, the arc is cooled and separated for extinction.
As described above, the circuit formed between the terminal plates (46) and (46) is opened and closed by controlling application of the driving voltage to the coil (78).
In the conventional electromagnetic contactor, the distance (X) between the lower part (72a) of the commutation plate (72) and the arc runner (56) and the distance (Y) between the lower part (70a) of the grids (70) placed adjacent to the commutation plate (72) and the arc runner (56) had relation of X&gt;Y as shown in FIG. 3. Arc produced between the stationary contact point (54) and the movable contact point (24) at the breaking operation of the electromagnetic contactor is separated by electromagnetic force caused by current passing through the movable and stationary contacts (20), (50): one is commutated from the movable contact point (24) to the commutation plate (72) and the other is transferred from the stationary contact point (54) to the arc runner (56) to elongate positive column of the arc thus breaking of current is performed by pulling the arc between the grids (70). Since there was the relation of X&gt;Y in the distance as described above, the elongated arcs on the commutation (72) and the arc runner (56) is contrary shortened to stick on the commutation plate (72), the grids (70) and the arc runner (56) whereby sometimes breaking can not be attained. Particularly, when breaking current is small, possibility of the fault is high because electromagnetic force caused by current passing through the movable and stationary contacts (20), (50) is small.
In the conventional electromagnetic contactor, the grid (70) has been attached to the arc box (12) such a manner that a pair of fitting pawls (70c) formed on the upper surface of the grid (70) are inserted into the slits (12a) formed in the arc box (12) to extend outside as shown in FIG. 4 and then each of the top is bent as shown in FIG. 5. Therefore, the top of the bent portion of the pawl (70c) comes to an adjacent grid whereby the creepage distance (1) between the adjacent projections is smaller than the creepage distance (L) of the adjacent grid to possibly cause flash over on the outer surface of the arc box.